The present invention relates to field effect transistors (FETS) using a hetero-structure of nitride semiconductors represented by the general formula InxGayAl1-x-yN (with 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61).
Semiconductors including gallium nitride, such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and indium aluminum gallium nitride (InAlGaN), have a high dielectric breakdown electric field strength, high thermal conductivity, and high saturated electron drift velocity, for example, so they are preferred as materials for high frequency power devices. More specifically, a so-called two-dimensional electron gas is formed in the heterojunction structure of an AlGaN film serving as the top layer and a GaN film serving as the bottom layer (hereinafter referred to as xe2x80x9cAlGaN/GaN hetero-structurexe2x80x9d), by accumulating a high concentration of electrons near the heterojunction interface in the GaN film.
This two-dimensional electron gas exhibits high electron mobility because it is spatially separated from the donor impurities added to the AlGaN film. Consequently, the source resistance component can be reduced by using an AlGaN/GaN hetero-structure in field effect transistors.
The distance d from the gate electrode formed on the AlGaN/GaN hetero-structure to the two-dimensional electron gas is usually short at about several dozen nm, so even if the gate length Lg is short at about 100 nm, the ratio Lg/d (aspect ratio) of the gate length Ld to the distance d can be increased to about 5 to 10. Therefore, an excellent characteristic of AlGaN/GaN hetero-structures is that they make it easy to fabricate field effect transistors with little short channel effect and good saturation properties.
Additionally, in high electric field regions of about 1xc3x97105 V/cm, the two-dimensional electron gas in the AlGaN/GaN hetero-structure has at least twice the electron velocity of currently available AlGaAs/InGaAs hetero-structures, for example, and therefore its application as a high frequency transistor material in high frequency power devices is anticipated.
However, one problem in field effect transistors using AlGaN/GaN hetero-structures or GaN is that there may be instabilities in transistor operations, depending on the approach used to apply the gate voltage or drain voltage. More specifically, it has been reported that the drain current decreases for other than thermal reasons when the drain voltage is increased, and also that the drain current gradually decreases when the strength or frequency of signals applied as the gate voltage is increased.
Drain current is thought to decrease for the following reasons:
(1) Poor crystal quality in the AlGaN film of the AlGaN/GaN hetero-structure leads to deep energy levels in the AlGaN film caused by numerous defects, with the deep energy levels acting as electron trapping centers (electron traps).
(2) Numerous defects in the surfaces of the GaN and the AlGaN films cause deep energy levels that contribute to the trapping and releasing of electrons.
On the other hand, a method for reducing the drop in drain current is to form a GaN film doped with n-type impurities in high concentration as a cap layer on the AlGaN/GaN hetero-structure, that is, on the AlGaN film.
FIG. 4A is a cross-sectional view of a conventional semiconductor device, or more specifically, a field effect transistor, using an AlGaN/GaN structure with this cap layer.
As shown in FIG. 4A, a buffer layer 13 made of a GaN film, and an electron supply layer 14 made of an n-type AlGaN film, are sequentially formed on a substrate 11 made of sapphire or silicon carbide (SiC) via an AlN (aluminum nitride) film 12. A cap layer 15 made of an n-type GaN film covers the upper surface of the electron supply layer 14. A gate electrode 16 is formed on the electron supply layer 14 within a recessed portion provided in a predetermined region of the cap layer 15, and a source electrode 17 and a drain electrode 18 are formed on either side of the gate electrode 16 on the cap layer 15.
In this conventional semiconductor device, a high-concentration two-dimensional electron gas 19 is formed in the buffer layer 13 near the interface with the electron supply layer 14 so that the semiconductor device can be operated as a FET by controlling the concentration of the two-dimensional electron gas 19 with the voltage applied to the gate electrode 16. That is, the upper portion of the buffer layer 13 functions as a channel layer.
In this conventional semiconductor device, the cap layer 15 protects the surface of the electron supply layer 14, so that the formation of deep energy levels caused by defects in the surface of the electron supply layer 14 can be inhibited. As a result, fluctuations in the potential energy of the electrons (hereinafter referred to simply as xe2x80x9cpotentialxe2x80x9d) caused by electrons being trapped and released at the surface of the electron supply layer 14 can be suppressed. At this time, adding n-type impurities to the GaN film serving as the cap layer 15 can increase the distance from the surface of the electron supply layer 14 to the two-dimensional electron gas 19, thereby lessening the effect that fluctuations in potential in the surface of the electron supply layer 14 have on the potential of the channel layer.
Conventional semiconductor devices, however, have drawbacks in that a reduction in drain current cannot be adequately prevented, and the contact resistance of the source electrode 17 and the drain electrode 18, which are ohmic electrodes, is increased.
In light of the above, it is an object of the present invention to reliably prevent a reduction in drain current so as to stabilize FET operations and to reduce the contact resistance of the ohmic electrodes in FETs using a hetero-structure semiconductor including GaN.
To achieve this object, the inventors assessed the problems with conventional semiconductor devices, namely, a first problem of inadequate prevention of a reduction in drain current, and a second problem of an increase in contact resistance of the ohmic electrodes.
As mentioned earlier, in order to reduce the effect that traps in the surface of the hetero-structure of a semiconductor including GaN have on operation of the FET, it is effective to increase the distance from the surface of the hetero-structure to the region in which the two-dimensional electron gas is formed, that is, to the channel layer of the FET. In other words, increasing this distance makes it possible to reduce the effect that fluctuations in the surface potential of the surface of the hetero-structure caused by the trapping and releasing of electrons have on the potential of the channel layer. However, in the case of an AlGaN/GaN hetero-structure, that is, when an AlGaN film is used as the electron supply layer, the AlGaN layer itself cannot be made thick for attaining this effect because the AlGaN film and the GaN film have different lattice constants.
Accordingly, in the conventional semiconductor device, the cap layer 15 made of the n-type GaN film is formed on the electron supply layer 14 made of the AlGaN film to achieve the above-described effect.
In assessing the first problem, the inventors found that in the conventional semiconductor device, the difference between the spontaneous polarization of the GaN film serving as the cap layer 15 and the spontaneous and piezoelectric polarization of the AlGaN film serving as the electron supply layer 14 results in a drop in electron concentration in the channel layer of the FET, thereby causing the first problem of inadequate prevention of the reduction in drain current.
Next, in assessing the second problem, the inventors found that in the conventional semiconductor device, a potential hill caused by the above difference in polarization between the GaN film and the AlGaN film is formed at the interface between the cap layer 15 and the electron supply layer 14, because an ordinary hetero-structure in which the surface is a c face of group III atoms is used as the AlGaN/GaN hetero-structure, thereby resulting in the second problem of increased contact resistance of the ohmic electrodes.
FIG. 4B is a diagram schematically illustrating the change in the potential energy of the electrons taken along the line A-Axe2x80x2 of FIG. 4A.
As shown in FIG. 4B, the effective contact resistance at the portions of the source electrode 17 and the drain electrode 18 that substantially function as ohmic electrodes, increases when the source electrode 17 and the drain electrode 18 are formed on the cap layer 15, because a potential hill occurs at the junction portion between the cap layer 15 (n-type GaN film) and the electron supply layer 14 (n-type AlGaN film).
The present invention was conceived in light of the above findings. More specifically, a first semiconductor device according to the present invention includes a GaN film formed on a substrate, an AlGaN film formed on the GaN film, a gate electrode formed on the AlGaN film, and source and drain electrodes formed on either side of the gate electrode on the AlGaN film; wherein an n-type InxGayAl1-x-yN film (wherein 0 less than x less than 1, 0xe2x89xa6yxe2x89xa61, 0 less than x+y less than 1) is formed between the source and drain electrodes and the AlGaN film.
According to this first semiconductor device, a source electrode and a drain electrode (hereinafter, also referred to as the source and drain electrodes) are formed on a hetero-structure of AlGaN/GaN, that is, on the AlGaN film, via an InGaAlN film (which can also be an InAlN film). Since the InGaAlN film has been doped with n-type impurities, the surface of the hetero-structure can be protected, so that it is possible to suppress the effect of deep energy levels caused by defects in the surface, thereby inhibiting fluctuations in the potential caused by the trapping and releasing of electrons at the surface. Also, by inserting the thick InAlGaN film, it is possible to increase the distance from the surface of the hetero-structure to the channel region, which is formed by the two-dimensional electron gas in the hetero-structure. Consequently, the effect that fluctuations in the potential at the surface of the hetero-structure have on the potential of the channel layer can be diminished, so a reduction in drain current can be reliably prevented, stabilizing the operation of the FET and increasing the output power of the FET.
According to this first semiconductor device, an InGaAlN film is used in place of the conventional GaN film as the cap layer of the hetero-structure, so it is possible to reduce the difference in polarization between the AlGaN film and the cap layer. Thus, the formation of a potential hill at the interface between the AlGaN film and the cap layer can be inhibited, so that it is possible to reduce the contact resistance of the ohmic electrodes when source and drain electrodes, which are ohmic electrodes, are formed on the cap layer. Consequently, the properties of the FET can be improved and the efficiency of the FET can be increased.
It is preferable that in the first semiconductor device, the composition of the InxGayAl1-x-yN film is set so that the lattice constant of the InxGayAl1-x-yN film and the lattice constant of the GaN film are substantially matching, and polarization occurring in the InxGayAl1-x-yN film is equal to or larger than polarization occurring in the AlGaN film.
Thus, the InGaAlN film serving as the cap layer can be formed thickly, so the distance from the surface of the hetero-structure to the channel layer can be further increased to reliably lessen the effect that potential fluctuations in the surface of the hetero-structure have on the potential of the channel layer. Furthermore, it is possible to make the slope of the potential between the AlGaN film and the cap layer substantially constant, or to form a potential valley, so that the contact resistance of ohmic electrodes formed on the cap layer can be reliably decreased.
In this first semiconductor device, it is also possible to form an InGaN film or a layered film of an InGaN film and another GaN film as a channel layer between the GaN film and the AlGaN film.
A second semiconductor device according to the present invention includes a GaN film formed on a substrate; an n-type InxGayAl1-x-yN film (wherein 0 less than x less than 1, 0xe2x89xa6y less than 1, 0 less than x+y less than 1) formed on the GaN film; a gate electrode formed on the InxGayAl1-x-yN film; and source and drain electrodes formed on either side of the gate electrode on the InxGayAl1-x-yN film.
According to this second semiconductor device, the source and drain electrodes are formed on the hetero-structure of a GaN film and an n-type InGaAlN film (which can also be an InAlN film). Thus, the InGaAlN film can be formed thickly so as to increase the distance from the surface of the hetero-structure to the channel region formed in the hetero-structure by the two-dimensional electron gas. Consequently, the effect that fluctuations in the potential at the surface of the hetero-structure have on the potential of the channel layer can be reduced, and therefore a reduction in drain current can be reliably prevented, stabilizing FET operations and increase the output power of the FET.
According to this second semiconductor device, forming the InGaAlN film thick makes it unnecessary to provide a cap layer on the hetero-structure. Thus, there are no potential hills formed at the interface between the hetero-structure and the cap layer due to a difference in polarization between semiconductor layers, which is the case when for example a conventional GaN film is formed as a cap layer on an AlGaN/GaN hetero-structure. Consequently, the contact resistance of ohmic electrodes can be reduced even when source and drain electrodes, which are ohmic electrodes, are formed on the InGaAlN film, and therefore the FET properties and the efficiency of the FET can be improved.
It is preferable that in the second semiconductor device, the composition of the InxGayAl1-x-yN film is set so that the lattice constant of the InxGayAl1-x-yN film and the lattice constant of the GaN film are substantially matching.
Thus, a thick InGaAlN film can be reliably formed, so the effect that fluctuations in the potential at the surface of the hetero-structure have on the potential of the channel layer can be reliably reduced.
In the second semiconductor device, it is also possible to form an InGaN film or a layered film of an InGaN film and another GaN film to serve as a channel layer between the GaN film and the InxGayAl1-x-yN film.